In a Dec. 30, 2013 photo, an ambulance leaves the site of a trolleybus bombing in the city of Volgograd. / Denis Tyrin, AP

by Lynn Barry, Associated Press

by Lynn Barry, Associated Press

MOSCOW (AP) - Six men are dead in a series of unexplained killings involving booby-trapped bombs in southern Russia, further heightening security fears ahead of next month's Winter Olympics in Sochi.

Investigators were scrambling Thursday to find those responsible for the six bodies found Wednesday in four abandoned cars just north of Russia's volatile Caucasus Mountains region, where an Islamic insurgency is simmering.

Explosive devices had been placed near three of the cars, although only one of the bombs went off and no one was hurt. The victims had been shot, according to investigators.

Vladimir Markin, the spokesman for Russia's main investigative agency, said in a statement that no motive had yet been found for the killings on the outskirts of Pyatigorsk, the center of a Russian administrative district created in 2010 to combat the insurgency. In late December, a car bomb exploded outside traffic police offices there, killing three people.

Pyatigorsk is less than 200 miles by air from Sochi, host site for the 2014 Olympics, although nearly twice as far by road.

In an indication of Russia's unease over security ahead of the Olympics, Markin said Federal Security Service officers had joined the investigation and classified it as a counter-terrorist operation.

The shootings of local residents - at least a few of them taxi drivers - is more typical of criminal behavior, perhaps score-settling by organized gangs. But the use of explosives was suggestive of the kinds of terror attacks that take place nearly daily in the Caucasus.

Russia is still on edge following two suicide bombings in late December in Volgograd, also in southern Russia, which killed 34 people and wounded many more. No claim of responsibility has been made for those bombings, but they came several months after the leader of the Islamic insurgency called for attacks aimed at undermining the games, which run Feb. 7-23.

NTV television, a national channel loyal to the Kremlin, showed photographs of four suspects that it said had been distributed to police. The men were said to be from Kabardino-Balkaria, just south of Pyatigorsk and one of the predominantly Muslim republics in Russia's Caucasus.

The NTV journalist on the scene said investigators believed the killings were of a criminal nature, but were not ruling out other motives.

Kommersant, a major Moscow-based newspaper, suggested the attacks were carried out by militants and the explosives were intended to harm police when they arrived to investigate. That tactic has been used before in Pyatigorsk, where a taxi driver was killed in 2010 and his car then used as a bomb, wounding more than 30 people.

Three of the men found Wednesday have been identified: Two were taxi drivers and the third assembled furniture but also worked as a freelance taxi driver, Russian state news agencies reported, citing law enforcement agencies. Their names have not been released.

The men drove inexpensive Soviet-model Lada cars. Homemade bombs were placed near two of their cars; one of them went off as police approached and the other was defused.

The three other victims were found in a fourth vehicle. An explosive device had been placed next to the car in a metal bucket, but was defused by investigators, Markin said. No information about their identities has been released.

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